Processes for the desalination of seawater, brackish water, or in general saline waters associated with the production of oil, gas, coal and other minerals have now been practiced on a large scale for more than 50 years. For many years, thermal technologies were the only viable option, and multi-stage flash (MSF) was established as the baseline technology. Multi-effect evaporation (MEE) may now vie for that status. With the growth of membrane science, however, reverse osmosis (RO) overtook MSF as the leading desalination technology, and is presently considered the baseline technology.
Among the numerous factors affecting the selection of a desalination process, the cost of energy overshadows all the others. While energy cost is not the only determining criterion for process selection, it is definitely one of great concern. Additional issues such as environmental footprint, chemicals consumption and discharge, maintenance, ease of operation, reliability, on-stream factor, safety, and overall cost of production will influence the selection.
Thermal systems are effective but energy intensive, requiring in their simplest form approximately 1000 btu/lb to vaporize water. To compensate, distillation systems use many stages to reuse the heat energy repeatedly, with intricate heat exchange networks.
In the last several years, Seawater Reverse Osmosis (SWRO) has realized substantial power reductions. In spite of these improvements, hailed by some as approaching the thermodynamic minimum power expenditure, the data published by ADC (Affordable Desalination Collaboration, California, USA) show that the overall power costs still represent approximately 45-55% of the total cost of production. Additionally, there are concerns with regard to environmental impact, maintenance, and on-stream time. In their totality, these factors prevent SWRO, in its present form, from being the optimal desalination choice. Reverse osmosis systems have steadily increased recovery rates and now have ways to recapture energy from the pressurized waste brine. Nevertheless, in spite of these forward strides, these systems have not yet attained the elusive goals of environmental friendliness, ease of operation, low maintenance, low operating costs, low investment, and long-term reliability desired and needed by a thirsty world. While research in SWRO continues and there is hope that the process can be further refined, the question must be asked whether the fascination with that process may have blocked out other worthy processes, such as, for example, freezing.
Freezing systems for the desalination of seawater created a lot of interest several decades ago. Freezing seawater produces pure ice that is salt free. That interest has waned in the face of successful innovations of other technologies that have the allure of being newer processes. Distillation and reverse osmosis systems are among these.
Freezing technology reached its high point in popularity and interest some fifty years ago. This was due to the inherent efficiency of the freezing process that requires merely one sixth of the energy when compared to simple distillation, requiring roughly 150 btu/lb to freeze water as compared to 1000 btu/lb to vaporize water, as may be required in distillation processes. The low operating temperature of the freezing process enhanced its attractiveness because it reduced corrosion, requiring less costly materials of construction. But its greatest allure, for both small and large-scale operation, was the fact the equipment components and designs for freezing had a long history of trouble free operation as evidenced by the many refrigeration installations within most industries all over the world.
The U.S. government funded desalination research when it established the Office of Saline Water (OSW). From the late 1950's to 1980, the OSW appropriated some $160 million to desalination research. Distillation processes had not fully blossomed to their present state. Membrane processes had not yet surfaced, though ultimately they emerged as part of this funding. Several variations of the freezing process were developed, and many systems have been proposed to achieve an economical salt-water freezing process (ice or hydrate). Some of these processes use a refrigerant to chill the saline waters to form an ice or hydrate slush by either direct contact of refrigerant with saline water or by indirect heat exchange. For example, the process in U.S. Pat. No. 3,213,633 uses direct heat exchange of a C1-C5 refrigerant, including fluoro- and chloro-carbons. Others use evaporative cooling to obtain ice, the water being evaporated under vacuum thereby inducing freezing of the seawater.
Because the money was available from the government, there was a rush to build large demonstration plants without the benefit of extensive piloting on a small scale. Approximately a dozen diverse freeze demonstration plants were constructed, some located at the OSW Wrightsville Beach Test Facility in North Carolina and one plant was built in St. Petersburg, Fla. Additionally, the OSW provided financial assistance to a demonstration plant built in Israel using the Zarchin Process. That plant operated for two years providing water to a nearby town.
All of the projects experienced major difficulties at first. After start-up, plant modifications needed to be made. Budgets and schedules fell to the wayside due to the required design changes, which on a large-scale are expensive and time consuming. Once these plants were operating, it became evident that harvesting the ice from the brine was a serious problem. Ice crystals form as flat platelets, of irregular shape, preventing the mother liquor brine from draining freely. Mother liquor adheres to the crystal surfaces and interstices. The brine and ice form a slush that resists proper separation. Upon melting, the resulting water contains the salt of the adhering brine.
Unfortunately, the problem of how to separate easily the ice from the residual brine proved to be an obstacle that detracted from the overall appeal of the freezing process. Desalination plants, using the freezing process, never expanded beyond small units, which at best were only pilot units under different names. The main limiting problem to a practical concept, suitable for large-scale operation, was the hurdle presented by the harvesting of ice, prior to its melting, to produce fresh water. Numerous means for separating ice and brine were tried including filtration, centrifuging or similar operations all of which yielded frustrating results. The most successful apparatus was a wash column, developed by Prof. Wiegandt at Cornell University, in which a solid column of ice is pushed upwards by hydraulic pressure. As the ice reaches the top of the column, a mechanical device, scrapes and cuts the ice, dropping it into a melter from which pure water product is withdrawn. Some of the product is recycled to the top of the column for washing. The brine leaves at the bottom of the column. This apparatus produces pure water. However, it is awkward and cannot be scaled up to large capacities.
Ice separation has been the greatest impediment in the development of the freezing process and has proven to be a limiting factor in the design of large capacity plants, to cause acute trouble spots in the process, to require constant supervision, labor and high maintenance. The problems encountered gave the freezing process a bad reputation, curtailing research in this area.